127 



water, ultra-thin sections (thickness < 500 A) in air (water and air are 

 better than glycerol jelly on account of their lower refractive index). The 

 following formula shows the importance of a wide difference between the 

 refractive index of the object and that of the embedding medium when 

 the geometrical thickness of the section is decreased: 



OPD = e{n^- n^; OPD, optical path difference (see p. 131); /Ji, re- 

 fractive index of the object; n.^, that of the enclosing medium; e, geometrical 

 thickness of the object. 



If air is used as an enclosing medium it is, of course, possible to apply 

 cover slips and immersion oil in the ordinary way. For theoretical reasons, 

 however, full use cannot be made of the numerical aperture of the micro- 

 scope. Good results have, nevertheless, been obtained by this method. When 

 specifying the thickness of the sections, e.g. ultra-thin sections embedded 

 in air, the figures originally obtained from electron micrographs of shadowed 

 sections (Sjostrand, personal communication) have been used. The present 

 author has measured the thickness of ultra-thin sections by means of 

 interference microscopy. These measurements, in general, tend to give 

 slightly higher values than those calculated from shadowed sections. 



It may perhaps not be superfluous to point to another circumstance that 

 might cause confusion. If thin exine sections are studied by means of an 

 ordinary light microscope a number of diffraction lines can usually be seen, 

 due to superposed light waves in the thin exine. The number of the waves 

 can be altered by changing the wave length of the light used (Ingelstam, 

 personal communication). The diffraction lines must not be mistaken for 

 lamellae or demarcation lines between different layers of the exine as has 

 apparently been done by Pflug in describing optical sections of entire grains 

 [cf. Pflug 1953, p. 66: "Exospore and Exinen sind aus konzentrischen 

 Schalen (Lamellae) zusammengesetzt; das ist in unzahligen Fallen be- 

 obachtet. Fraglich ist nur deren Zahl. Sie ist einmal subjektiv bedingt durch 

 die Scharfe der Optik"]. 



Ultra-thin sections should always be properly stained in order to obtain 

 as much information on wall stratification, cell contents etc. as possible. 

 The fact that the immature wall of certain fungus spores is easily penetrated 

 by stains (Nannfeldt, personal communication) seems to open up a pos- 

 sibility for obtaining further details on sporoderm formation. Further 

 research work in sporoderm morphogenesis is indeed necessary. 



In this context another problem may be mentioned: electron micrographs 

 of acetolyzed exines often show that the exine, the endonexine excepted, 

 is altogether homogeneous-amorphous (cf. e.g. EMG by Afzelius in Erdt- 

 man, 1956a, Fig. 1, p. 132). No demarcation lines can be observed at a 

 resolution of 40-50 A. Oshurkova (1956) in a review of a paper on the fine 



